Nitrogen use efficiency and/or fertilizer
recovery in crop production systems can be computed using many different
methods. Those specifically designated for crop production systems follow.
In addition, nitrogen use efficiency has been estimated using world N
consumption and crop production, but on macro scales (Agron.
J. 91:357-363). Other methods (Difference Method, Isotopic Method
(Enriched and Depleted)) follow below. Finally, the components of Nitrogen
Use Efficiency are also discussed, initially discussed by Moll et al.
(1982). Regardless of the method, almost all result in estimated NUE for
cereal production from 30 to 35%. Topdress or sidedress N applications in
the middle of the season can result greater NUE's (>50%). Because the risk
of N loss is greater with fall N application, N should be applied in the
spring to minimize risk and optimize profitability regardless of tillage
system (Vetsch and Randall, 2004).

15N composition of the total N of grain and
leaf samples of corn (Zea mays L.) decreased systematically as N fertilizer
rates increased (Kohl et al., 1973). This result was considered to be consistent
with increasing contributions of fertilizer N to plants as the rate of applied N
increased.

Hauck and Bremner, 1976

percent nitrogen recovered (plant or soil) =

= 100P (c-b) / f(a-b)

P = total N in the plant part or soil in kg
ha-1
f = rate of 15N fertilizer applied
a = atom percent 15N in the labeled fertilizer
b = atom percent 15N in the plant part or soil receiving no 15N
c = atom percent 15N in the plant part or soil that did receive 15N